Position aided subscriber unit for a satellite cellular system

ABSTRACT

A subscriber unit for communicating with a satellite communication system. This subscriber unit receives and transmits digital data from satellites of a satellite communication system. In addition, the subscriber unit self-determines its own position and transmits this position to the satellites of the satellite communication system. In this way, the satellite communication sytem knows the position of each subscriber unit so that communications may be established more efficiently. In addition, the position of the subscriber unit is automatically determined from the received GPS signals and reported to the satellite communication system for other uses such as asset management and search and rescue operations.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present case is related to co-pending U.S. applications Ser. No.263,849 abandoned; Ser. No. 402,743 abandoned which has the sameinventive entity as the present application; Ser. No. 415,814 abandoned;Ser. No. 415,842 pending; Ser. No. 415,815 pending; and Ser. No. 414,494pending all being assigned to the same assignee as the presentinvention.

BACKGROUND OF THE INVENTION

The present invention pertains to portable communication devices andmore particularly to a position aided, portable communication device foruse with a satellite communication system.

When achieving communication between satellites and users of a satellitecommunication system around the world, knowledge of the location of eachuser is important to establishing and maintaining the communicationlink. Knowledge of the location of a particular user is also importantbecause that location information may be reported through the satellitelink which serves the user to any other user worldwide. A single unitwhich combines voice/data communication and precise locationdetermination has several advantages over the present day communicationsystem devices. Applications of such a system include asset managementas well as coordination of activities of search and rescue and variousmilitary operations.

Since communications of the user unit to satellites involve frequenthand-offs between cells created by individual satellites and betweenadjacent satellites, the knowledge of precise location of both the userunit and of the satellites can be used to efficiently determine anappropriate hand-off strategy.

Further, precise knowledge of the location of a user unit also aids inthe acquisition process by permitting precise correction for Doppler andreference frequency induced errors. The satellites of a low-earth orbitsatellite communication system such as Iridium move very rapidlycompared with the location of a user on earth. During communications,the system must provide Doppler compensation to the signals tocompensate for the rapidly changing propagation delay in the link.During acquisition of the signals, if the user unit cannot independentlydetermine the Doppler frequency, an exhaustive frequency search of theDoppler range must be performed. This length of time for an exhaustivesearch would delay the recognition of the user by the system. Therefore,in the case of telecommunication, the user would experience a delay inbeing able to establish communications.

However, the communication satellite ephemerides can be stored in theuser unit and will remain valid for several days. Within knowledge ofthe precise location of the user unit, the Doppler adjustment as well asthe correction to the reference oscillator can be explicitly computed.This would permit more rapid acquisition and tracking of the satellitepilot channel.

Accordingly, it is an object of the present invention to provide acombined position aided voice/data communicator device for use with asatellite communication system.

SUMMARY OF THE INVENTION

In accomplishing the object of the present invention, a novel positionaided voice/data subscriber unit for a satellite cellular system will beshown.

A satellite communication system includes a number of subscriber units,each subscriber unit for receiving and transmitting digital data. Inaddition, each subscriber unit self-determines its position.

The subscriber unit includes a receiver/transmitter for receiving fromand transmitting to a satellite of the satellite communication systemdigital user data. In addition, the subscriber unit includes a receiverfor self-determining the position of the subscriber unit andtransmitting that position to the satellite of the satellitecommunication system.

The above and other objects, features, and advantages of the presentinvention will be better understood from the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The single sheet of drawings included herewith is a block diagram of acombined position aided subscriber unit for a satellite cellular system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The disclosure of co-pending U.S. patent applications Ser. Nos. 263,849;402,743; 415,814; 415,842; 415,815; and 414,494 are hereby incorporatedby reference.

Referring to the single sheet of drawings included herewith, antenna 1is shown. Antenna 1 receives information from a global positioningsystem (GPS), not shown, as well as information transmitted from thesatellites of a satellite communication system (not shown). In addition,antenna 1 operates to transmit voice and data information from thesubscriber to the satellite communication system.

Antenna 1 is connected to circulator 2 which provides isolation betweenthe transmitted and the received signals. Circulator 2 operates suchthat the incoming signals received from antenna 1 are transmitted todiplexer filter 3 and outgoing signal from transmitter/modulator 12 istransmitted from transmitter/modulator 12 to antenna 1 for outgoingtransmission.

Diplexer filter 3 is connected to circulator 2. Diplexer filter 3includes amplification and operates to pass both the GPS signals andsatellite transmitted signals for subsequent processing. Diplexer 3 isfurther connected to mixer 4 and to GPS receiver 9 via path 10.

Since the GPS information as well as the transmissions from thesatellite are in the same frequency band, they must be separated forprocessing by the communication system.

The output of mixer 4 is connected to signal discriminator 5. Signaldiscriminator 5 separates the satellite communication signals from allother signals received by the system. The satellite communicationsignals are in the L-band which is approximately 1.5 to 1.6 GHz. Signaldiscriminator 5 strips data from the carrier after determining theappropriate tracking code.

Local oscillator 6 is connected to signal discriminator 5, to digitalsynthesizer 8 and to GPS receiver 9. Local oscillator 6 is a referencefrequency source. Signal discriminator 5 detects a reference clock errorand is connected to processor 7 analogous to a phase lock loop. Sucherror is interpreted by the processor to adjust the Digital SynthesizerFrequency in such a manner to maintain phase with the signal modulation.Processor 7 is connected to digital synthesizer 8 and digitalsynthesizer 8 is connected to mixer 4 in a feedback loop configuration.Data transmitted from the satellite is sent at 2 millisecond bursts(approximately 14 MHz data stream) at a 30 millisecond rate. The signaldiscriminator 5 is based on bit correlation and recognizes apredetermined header pattern. Information following the header is thedesired communication information transmitted by the satellite system.

The signal discriminator transmits bits of information representing thecommunication information to processor 7. Processor 7 derives a measureof the phase error which is the difference in phase between the bits ofthe predetermined header pattern synthesized in the synthesizer and thecorresponding bits modulated on the satellite signal. As a result,processor 7 determines whether the bits are being received early or lateand adjusts digital synthesizer 8 to transmit an adjusted referencesignal to mixer 4 for synchronizing receipt of the satellitecommunication information. Processor 7 determines the adjustment inputto the digital synthesizer 8 from the nominal reference of the localoscillator 6. Since there is rapid movement of the satellites of thecommunication system, the received signals at L-band may vary over arange of approximately 70 KHz. Local oscillator 6 has an additional 3KHz uncertainty as well. Compensation for these variations are performedby adjustment in the digital synthesizer 8 which account for each ofthese variations of the signals and the local oscillator.

The processor 7 would obtain measurements from the signal discriminator5 while it is demodulating the data from the communication link thatwould alter the reference signal frequency provided by the localoscillator 6 so that the input signal to the mixer 4 would perfectlymatch the received signal frequency output from the diplexer 3 and inputto mixer 4. The offset frequency so derived by the processor 7 andcommanded to the signal synthesizer 8 is mathematically represented as asum of the reference frequency error and the Doppler shift frequencysignal from the communication satellite system (not shown) to thereceiver antenna 1.

GPS signals received from the global positioning system satellites arereceived by antenna 1 and transmitted through circulator 2 and throughdiplexer 3 to GPS receiver 9 via connection 10. The GPS system includesa group of at least four satellites from which time and locationinformation may be derived. From this time and location information, aGPS receiver may calculate its own position. GPS receiver 9 operatesupon the data received via antenna 1, circulator 2, and diplexer filter3 to perform these functions for the subscriber unit. The GPS satellitesare in 12-hour orbits at approximately 22,000 kilometers above theearth. GPS receiver 9 determines a pseudo range for each of the foursatellites of the GPS system. GPS receiver 9 then accurately calculatesthe coordinates of the corresponding subscriber unit. The processor 7has stored in an associated memory all the ephemerides of the satellitesof the communication system. As a result, processor 7 knows whichsatellite is overhead and at which time to initiate a messagetransmission to the satellite so that it will fall into an appropriateTime Division Multiple Access time slot of the satellite receiver. Also,the processor 7 must adjust the signal for Doppler effects, which areattributed to the satellite's motion relative to the user during theinterval of communication.

The GPS receiver 9 tracks the signal from the GPS satellites appearingin the signal output from diplexer filter 3. These measurements togetherwith the reference of the local oscillator 6 define the true error inthe reference frequency defined by the local oscillator 6. The processor7 uses this estimate of the true error in reference frequency tocompensate the above-derived offset frequency and defined the Dopplercorrection uplink frequency from the local oscillator 6 using thedigital synthesizer 11 and the transmitter/modulator 12 for transmissionto the communication system via antenna 1. This signal will be at astandard frequency independent of the Doppler effect of relative motionof the satellite with respect to the terrestrial user unit. By using theGPS receiver 9 for defining a precise reference frequency, the downlinkDoppler shift can be distinguished from the effect of the receiver localoscillator 6 offset and is then used to synthesize a Doppler compensatedsignal for the uplink.

Processor 7 is connected to digital synthesizer 11. Digital synthesizer11 is connected to transmitter/modulator 12. Transmitter/modulator 12 isconnected to circulator 2. User data 13 in the form of digital voice ordigital data is supplied to the transmitter/modulator 12 by the user.

At the appropriate time in the telecommunications transmission,processor 7 will provide position information for including in themodulated signal format that is generated by the transmitter/modulator12. Transmitter/modulator 12 will convert the data to analog signals andtransmit it through circulator 2, to antenna 1 where it will be outputand transmitted to the appropriate satellite.

Once the telecommunication link has been established between thesubscriber device and satellite, user data 13 is transmitted totransmitter/modulator 12 for transmission to the satellite in theappropriate time slot. Under the control of processor 7, digitalsynthesizer 11 provides a carrier waveform having the appropriatefrequency, properly Doppler compensated and time synchronized at thesatellite receiver. As a result, the satellite receives a string of databeing sent from a plurality of users each having their respectiveinformation in the appropriate time slot.

The previously described subscriber unit for a satellite communicationsystem provides the following advantages. First, automatic worldwidetracking of a satellite user may be accomplished via the subscriberunit. This function is important for applications which keep track of aspecified element or person that is using the satellite communicationsystem. The primary use, of course, for the location detection by thesubscriber unit is for the communication system itself. Without suchinformation, the system would have to search over the entire world tofind a user. However, by reporting in occasionally, the subscriber unitlets the system know where it is so that the system resources may beused more efficiently when a call comes to that particular user.

Second, improved efficiency of each subscriber unit is achieved by thefast acquisition of the satellite pilot channel. However, prior totracking this signal, the subscriber unit must be able to correct forDoppler effects caused by the motion of the high-speed satellitesrelative to the user. By "remembering" the satellite system'sephemerides, the subscriber unit can predict the signals from each ofthe satellites and thus acquire the signals more quickly than if searchmethods were employed. In addition, uplink synchronization may be aidedby knowledge of range to the satellite. This information can beimmediately processed with knowledge of both the satellite's positionand the subscriber unit's position.

Third, call hand-offs between cells of a particular satellite or betweensatellites are dependent upon knowledge of position of the user relativeto the satellite locations. This information can be explicitlydetermined from knowledge of the subscriber unit's position and thesatellite's position. The communication satellite position can becomputed from stored ephemerides of the satellites which are acquiredfrom earth-based systems to which the satellites communicate andtransmitted to the user units along with other data on the downlink.Precise time is available on the satellites of the communication systemthrough the use of Rubidium reference clocks onboard each satellite. Thesubscriber unit position would be available as a primary output of theGPS receiver that is built in to the subscriber unit.

Although the preferred embodiment of the invention has been illustrated,and that form described in detail, it will be readily apparent to thoseskilled in the art that various modifications may be made thereinwithout departing from the spirit of the invention or from the scope ofthe appended claims.

What is claimed is:
 1. In a satellite communication system, a subscriberunit for receiving and for transmitting digital data and forself-determining the location of said subscriber unit, said subscriberunit comprising:means for receiving from and transmitting to a satelliteof said satellite communication system digital user data; said means forreceiving and transmitting including transmitter/receiver means coupledto a plurality of satellites of said satellite communication system andcoupled to a plurality of satellites of a global positioning system toreceive location information from said global positioning system toreceive and to transmit digital data to and from said satellites of saidsatellite communication system; and means for self-determining saidlocation of said subscriber unit from said location information of saidglobal positioning system and for transmitting said location of saidsubscriber unit to said satellite communication system, said means forself-determining being coupled to said transmitter/received means.
 2. Asubscriber unit as claimed in claim 1, wherein said transmit/receivemeans includes:antenna means coupled to said plurality of satellites ofsaid satellite communication system and coupled to said plurality ofsaid satellites of said global positioning system; and circulator meansconnected to said antenna means for transmitting signals simultaneouslyas signals are received from said plurality of said satellites of saidglobal positioning system.
 3. A subscriber unit as claimed in claim 2,wherein said transmit/receive means further includes:filter meansconnected to said circulator means, said filter means transmitting thosesignals in particular frequency ranges of signals transmitted by saidsatellites of said satellite communication system and of said satellitesof said global positioning system; and mixer means connected to saidfilter means, said mixer means for multiplying said signals receivedfrom said satellite of said satellite communication system.
 4. Asubscriber unit as claimed in claim 3, wherein said means for receivingand transmitting further includes means for recovering said digital datafrom said signals of said satellite communication system, said means forrecovering being connected to said mixer means.
 5. A subscriber unit asclaimed in claim 4, wherein said means for recovering furtherincludes:signal discriminator means connected to said mixer means, saidsignal discriminator means for determining a plurality of bitsrepresenting communication information of said satellite communicationsystem; processor means connected to said signal discriminator means,said processor means operating to recover and store said communicationinformation; digital synthesizer means connected between said processormeans and said mixer means, said digital synthesizer means operating toadjust said mixer means to synchronize said reception of said signals ofsaid satellite communication system; and local oscillator meansconnected to said signal discriminator means and to said digitalsynthesizer means, said local oscillator means for providing a referencesignal of a particular frequency.
 6. A subscriber unit as claimed inclaim 5, wherein said means for self-determining includes GPS receivermeans connected to said processor means, said local oscillator means,and to said filter means, said global positioning system receiver meansoperating in response to said signals of said satellites of said globalpositioning system to determine said location of said subscriber unitand to transmit said location to said processor means.
 7. A subscriberunit as claimed in claim 6, wherein said means for receiving andtransmitting further includes transmit means connected between saidprocessor means and said circulator means, said transmit means operatingto send digital data representing user data and digital datarepresenting said location of said subscriber unit.
 8. A subscriber unitas claimed in claim 7, wherein said transmit meansincludes:transmitter/modulator means connected to said circulator means;second digital synthesizer means connected to said local oscillatormeans, to said processor means and to said transmitter/modulator means;and said processor means synchronizing the transmission of said userdata and said location information via said digital synthesizer meansand said transmitter/modulator means to a particular TDMA satellite timeslot.
 9. A subscriber unit as claimed in claim 8, wherein there isfurther included means for generating user data, said means forgenerating user data connected to said transmitter/modulator means, saidtransmitter/modulator means operating to control the transmission ofsaid user data through said circulator means to said antenna means. 10.In a satellite communication system, a subscriber unit for receiving andfor transmitting digital data and for self-determining the location ofsaid subscriber unit, said subscriber unit comprising:means forreceiving from and transmitting to a satellite of said satellitecommunication system digital user data; said means for receiving andtransmitting further operating to receive digital data from a pluralityof satellites of a global positioning system; means for recovering theuser data transmitted by said satellite of said satellite communicationsystem; global positioning system receiver means for self-determiningsaid location of said subscriber unit from said digital data of saidsatellites of said global positioning system and for transmitting saidlocation of said subscriber unit to said satellite of said satellitecommunication system; and means for transmitting digital user data fromsaid subscriber unit to said satellite of said satellite communicationsystem.